The
Cartridge Magazine Control Regulations (
SOR/98-462 Part 4 Section 3), Sub-Section (4):
A cartridge magazine described in subsection (1) that has been altered or re-manufactured so that it is not capable of containing more than five or ten cartridges, as the case may be, of the type for which it was originally designed is not a prohibited device as prescribed by that subsection
if the modification to the magazine cannot be easily removed and the magazine cannot be easily further altered so that it is so capable of containing more than five or ten cartridges, as the case may be.
Sub-Section (5) gives a listing of accepted methods which qualify as permanent modifications, and they generally include welding, brazing, epoxy, ‘or any other similar method.’ Interesting to note that rivets are not mentioned, and all alterations given are applied to the magazine body, not the follower or other internal parts; I suppose those fall under similar methods.
But in your case the simplest would be Paragraph (a) under (5), which states ‘the indentation of its casing.’ If you can put a big ol' dent in the extension tube at the correct spot, that would seem to satisfy the Law.